1. Field of the Invention
The present invention relates to a ball screw apparatus which is used in industrial machines and the like.
2. Description of the Related Art
In a conventional ball screw apparatus of this type, a screw shaft includes a screw groove formed in the outer peripheral surface thereof and extends in the axial direction of the ball screw apparatus, a nut includes a screw groove formed in the inner peripheral surface thereof so as to correspond to the screw groove of the screw shaft, the nut is fitted with the screw shaft, and the screw groove of the nut and the screw groove of the screw shaft are opposed to each other to thereby form a spiral-shaped load raceway between them. In the load raceway, there are disposed a large number of balls in such a manner that they are able to roll and, due to rotation of the screw shaft (or nut), the nut (or screw shaft) can be moved in the axial direction of the ball screw apparatus through the rolling movements of the balls.
By the way, when the nut (or screw shaft) moves in the axial direction, the balls move while they are rolling along the spiral-shaped load raceway formed by the two screw grooves; however, in order to make the nut (or screw shaft) move continuously, the balls must be circulated endlessly.
As a method for circulating the balls endlessly, there are generally known a method of a circulation tube type and a method of an end cap type. In a ball screw apparatus of a compact type, in the nut, there is formed a ball return passage which penetrates through the nut in the axial direction thereof and, at the same time, a cut-away portion is formed in the end face of the nut, and a resin-made circulation piece including a ball circulation passage allowing the ball return passage and the load raceway formed between the two grooves is fitted with and fixed to the cut-away portion, whereby the load raceway, ball return passage and ball circulation passage cooperate together in forming an endless circulation passage for the balls.
Also, to fix the resin-made circulation piece to the end face of the nut, there is employed a method for screwing a setscrew from the end face side of the nut in the axial direction to thereby fix the circulation piece, or a method for setting a snap ring fitted into the inner peripheral portion of the end portion of the nut on a stepped portion formed on the nut end face side of the circulation piece to thereby fix the circulation piece.
However, in the above-mentioned conventional ball screw apparatus, since the resin-made circulation piece is fixed by screwing the setscrew from the end face side of the nut in the axial direction or by setting the snap ring fitted into the inner peripheral portion of the end portion of the nut on the stepped portion formed on the nut end face side of the circulation piece, there arise the following problems. That is, when the circulation piece is expanded and is then shrunk due to heat, in the case of the fixing method using the setscrew, the setscrew is loosened and, in the case of the fixing method using the snap ring, the circulation piece is shrunk in the axial direction; and, therefore, the circulation piece is shifted in the axial-direction position thereof. This shifts the position of the portion (that is, the leading end portion of the tongue portion) of the circulation piece where the balls are scooped up from the load raceway formed between the two screw grooves, or generates a clearance in the boundary portion between the ball circulation passage of the circulation piece and the ball return passage of the nut to thereby impede the smooth rolling motion of the balls, which lowers the reliability of the ball screw apparatus.
Also, in the case of the method in which the snap ring fitted into the inner peripheral portion of the end portion of the nut is set on the stepped portion formed on the nut end face side of the circulation piece, since the snap ring is simply placed on the stepped portion, when the circulation piece is shifted in position in the axial direction, there is a possibility that the snap ring can be removed out of position.